This invention relates generally to storage tanks having roofs that float on the surface of the stored product, and more particularly to secondary seals used in such tanks.
Floating roof tanks are widely used to store volatile petroleum-based liquids and limit the quantity of product evaporative emissions that may escape to the environment. Such tanks may be configured either as internal floating-roof tanks or as external floating-roof tanks. In each configuration, the floating roof is designed to remain in contact with the liquid surface of the product and to cover all of the surface of the product except for a small annular surface area between the outermost rim of the floating roof and the inside surface of the tank shell. A single primary rim seal may control product evaporative emissions from this annular area. However, for increased effectiveness, emissions from this annular area are conventionally controlled by a combination of perimeter rim seals, including a primary seal with a secondary seal mounted in the rim space above it.
Primary seals conventionally take the form of a piece of fabric extending between the floating roof and a shoe plate that bears on the tank shell. Examples of such seals are illustrated in Wagoner, U.S. Pat. No. 5,036,995 and in Ford et al, U.S. Pat. No. 5,529,200. Alternatively, primary seals may be in the form of resilient liquid- or foam-filled seals that are supported from the floating roof.
Allen et al., U.S. Pat. No, 2,536,019, discloses a seal that is spring-loaded and supported from the top of the floating roof pontoon. In addition to problems arising from stress on the seal structure when the illustrated shoe plates move vertically as the roof moves laterally, there are basic problems with this design due to interaction between the primary shoe and the closely-mounted tip seal.
By current EPA definition, secondary seals for floating-roof tanks span the entire distance between the floating roof and the tank shell, above a primary seal. Most conventional secondary seals are mounted to the floating roof and extend upwards across the annular rim space to contact the tank shell some vertical distance above the floating roof. The vertical distance represents a characteristic clearance requirement for the secondary seal.
One prevalent type of secondary seal includes metal compression plates that attach to the floating roof and support a tip seal against the tank shell, as disclosed in Kinghom et al., U.S. Pat. No. 4,116,358; Grove et al., U.S. Pat. No. 4,615,458; and Thiltgen et al., U.S. Pat. No. 4,308,968. In each of these designs, the compression plates are mounted at an angle to the tank shell.
The angle of the compression plates is critical. If the angle is too steep, the tip seal can become jammed against the tank shell as the seal attempts to pass over weld seams or other surface irregularities on the tank shell. If the angle is too shallow, the tip seal can drag against the tank shell or catch on a weld seam or other shell discontinuity. Either event may cause the compression plates to fold into the rim space and damage one or more sections of the secondary seal, opening gaps between the tip seal and the tank shell that can lead to increased evaporative emissions to the atmosphere.
Further, as a floating roof drifts toward one section of the tank shell, the angle of the compression plates becomes more vertical, increasing the vertical clearance required to keep the tip seal inside the tank and in contact with the tank shell. For a typical storage tank with a nominal 8″ rim space, the width of the rim space at any particular point may actually vary between about 4″ to more than 12″ as the roof moves, increasing the vertical clearance requirement to as much as 24″. Tank size or tank foundation considerations may also dictate a 10-inch or even 12-inch nominal width for the rim space, with permissible variations as large as ±7 inches or more. Consequently, the vertical clearance requirement for conventional secondary seals may sometimes exceed 31″.
This vertical clearance requirement presents a problem both for new tanks and for retrofitting old tanks. New tanks must be designed with excess, unusable capacity to account for the required vertical clearance, adding to the construction cost. Similarly, when a secondary seal is added to an existing floating-roof tank, the maximum filling height of the tank may need to be reduced to accommodate the required vertical clearance for the secondary seal. Any such reduction of the maximum filling height represents lost inventory to the owner/operator of the tank. For example, when a secondary seal is added to an existing 100-foot (≈30 meter) diameter floating-roof tank, a nominal 2-foot (0.6 meter) reduction in filling height represents a loss of approximately 117,500 gallons (2800 Bbl) of product storage. Such a loss can significantly reduce the revenue of the owner/operator of the tank.
Hills et al., U.S. Pat. No. 4,339,052, discloses a secondary seal in the form of a tube that is connected near the top of the floating roof. One problem with this arrangement is that the secondary seal can rotate upwards, out of the rim space as the floating roof descends during product send-out operations. Petri et al., U.S. Pat. No. 5,284,269, discloses a space-saving double-seal system comprised of two shoe segments mounted above each other. One problem with this arrangement is that the shoe supports of the primary seal extend beneath the floating roof, increasing the risk of interference with equipment inside the tank.
The tip seal in the Allen patent is mounted directly to the top of the shoe plate. Although this arrangement would require a minimum vertical clearance, the tip seal does not extend all the way to the floating roof and thus does not, under EPA guidelines, constitute a true secondary seal. Further, connecting the tip seal directly to the top of the shoe plate creates a risk that movement over girth seams, etc. that may cause the shoe plate to separate from the tank shell might also cause the tip seal to separate from the tank shell.
None of these seal configurations have found significant commercial success.
Because of disadvantages in previously-disclosed low-profile secondary seals, it is believed that there is a need for a new low-profile secondary seal that can be used to increase the storage capacity of existing floating-roof tanks currently equipped with conventional primary seals. Gallagher, U.S. Pat. No. 6,354,488 presents a low-profile secondary seal that can be used with a conventional primary seal utilizing shoe plates and a fabric seal. The tip seal is held against the tank shell by a resilient tube seal. While this low-profile secondary seal reduces the clearance required, there are alternative methods that will maintain tip pressure, possibly over a wider operating range with a lower clearance requirement.